1. Field of the Invention
The present invention relates to frequency conversion of electromagnetic (EM) signals. More particularly, the present invention relates to reducing or eliminating DC offset voltages when down-converting a signal in a communication system.
2. Background Art
Electromagnetic (EM) information signals (baseband signals) include, but are not limited to, video baseband signals, voice baseband signals, computer baseband signals, etc. Baseband signals include analog baseband signals and digital baseband signals.
It is often beneficial to propagate EM information signals at higher frequencies. This is generally true regardless of whether the propagation medium is wire, optic fiber, space, air, liquid, etc. To enhance efficiency and practicality, such as improved ability to radiate and added ability for multiple channels of baseband signals, up-conversion to a higher frequency is utilized. Conventional up-conversion processes modulate higher frequency carrier signals with baseband signals. Modulation refers to a variety of techniques for impressing information from the baseband signals onto the higher frequency carrier signals. The resultant signals are referred to herein as modulated carrier signals. For example, the amplitude of an AM carrier signal varies in relation to changes in the baseband signal, the frequency of an FM carrier signal varies in relation to changes in the baseband signal, and the phase of a PM carrier signal varies in relation to changes in the baseband signal.
In order to process the information that was in the baseband signal, the information must be extracted, or demodulated, from the modulated carrier signal. However, because conventional signal processing technology is limited in operational speed, conventional signal processing technology cannot easily demodulate a baseband signal from higher frequency modulated carrier signal directly. Instead, higher frequency modulated carrier signals must be down-converted to an intermediate frequency (IF), from where a conventional demodulator can demodulate the baseband signal.
Conventional down-converters include electrical components whose properties are frequency dependent. As a result, conventional down-converters are designed around specific frequencies or frequency ranges and do not work well outside their designed frequency range.
Typically, down-converters are included in receiver channels that include amplifiers and/or filters. When a received modulated carrier signal is relatively weak, as in, for example, a radio receiver, conventional down-converters will include additional amplifiers in the receive signal path. These amplifiers, down-converters, and other components in a receiver channel create unwanted DC offset voltages. Any DC offset in the receiver channel has the effect of competing with the signal of interest, producing a statistical bias much like an interference. Furthermore, the DC offset may cause the receiver channel to become saturated, such that a voltage rail is reached or exceeded. For example, DC offset may saturate a receiver channel when it is amplified by DC coupled gain amplifiers in the receiver channel at baseband. In such situations, the information signal may not be recoverable at all. Hence, it is desirable to reduce or entirely eliminate unwanted DC offset voltages from receiver channels. Furthermore, the DC offset voltages must be removed without distorting the signal of interest.